PROJECT SUMMARY Kaposi sarcoma-associated herpesvirus (KSHV) or human herpesvirus-8 (HHV-8), belongs to the gamma subfamily of human herpesviruses and is responsible for several human malignancies developed in individuals infected with human immunodeficiency virus-1 (HIV-1) or AIDS patients. While the incidence of KSHV diseases has significantly declined since the introduction of highly active antiretroviral therapy (HARRT), Kaposi Sarcoma (KS) remains to be the major type of cancer worldwide in people living with HIV and AIDS. KSHV is primarily transmitted through saliva in endemic regions and frequently results in the development of KS in the oral cavity of AIDS patients. Like all viruses, the ability of KSHV to manipulate host gene expression is vital for its survival. Recently we discovered a novel post-transcriptional mechanism encoded by KSHV to accomplish this. KSHV open reading frame 10 (ORF10) inhibits nuclear export of cellular mRNAs without affecting its own transcripts. This export inhibitory function of ORF10 requires the interaction with an RNA export factor, Rae1. Furthermore, we showed that export inhibition of ORF10 is selective for a subset of mRNAs, and that the 3' untranslated regions of the targeted genes confer transcript sensitivity to ORF10-mediated export inhibition. ORF10 of a closely related rodent virus, murine gammaherpesvirus 68 (MHV-68), also interacts with Rae1 and is responsible for inhibiting nuclear export of mRNA during MHV-68 infection. The conservation of protein interaction and protein function not only underscores the importance of regulating RNA export during gammaherpesvirus infection, but also argues for MHV-68 as an in vivo model to study ORF10. In this proposal, we will elucidate the molecular mechanisms of ORF10-mediated selective RNA export inhibition. The result may reveal a novel cellular regulation of mRNA export. Furthermore, we will determine the functional significance of ORF10 mRNA export inhibition during gammaherpesvirus infection. We will employ the KSHV and MHV-68 ORF10 mutants to identify ORF10-specific changes on host gene expression in the context of infection. Finally, we will define the in vivo role of ORF10 with mouse infection of the MHV-68 ORF10 mutant. These experiments will increase our understanding of how gammaherpesviruses hijack cellular mRNA export pathway to facilitate their own replication. Moreover, the study may uncover cellular functions critical for KSHV replication and provide insights into new therapeutic strategies for KSHV-associated diseases.